Nav1.1 dysfunction in genetic epilepsy with febrile seizures-plus or Dravet syndrome

Abstract

Relatively few SCN1A mutations associated with genetic epilepsy with febrile seizures plus (GEFS+) and Dravet syndrome (DS) have been functionally characterized. In contrast to GEFS+, many mutations detected in DS patients are predicted to have complete loss-of-function. However, functional consequences are not immediately apparent for DS missense mutations. Therefore, we performed biophysical analysis of three SCN1A missense mutations (R865G, R946C, and R946H) we detected in six patients with DS. Furthermore, we compared the functionality of the R865G DS mutation with that of a R859H mutation detected in a GEFS+ patient; both mutations reside in the same voltage sensor domain of Nav1.1. The four mutations were co-expressed with β1 and β2-subunits in tsA201 cells and characterized using the whole-cell patch clamp technique. The two DS mutations, R946C and R946H, were non-functional. However, the novel voltage sensor mutants R859H (GEFS+) and R865G (DS) produced sodium current densities comparable to wild-type channels. Both mutants had negative shifts in the voltage dependence of activation, slower recovery from inactivation, and increased persistent current. Only the GEFS+ mutant exhibited a loss-of-function in voltage dependent channel availability. Our results suggest that the R859H mutation causes GEFS+ by a mixture of biophysical defects in Nav1.1 gating. Interestingly, while loss of Nav1.1 function is common in DS, the R865G mutation may cause DS by overall gain-of-function defects.

Naturally Occurring Carboxypeptidase A6 Mutations

“…However, because all of the other mutants were examined using only one substrate, it remains a possibility that some mutations in CPA6 alter the selectivity of the enzyme. Both gain-of-function (31,32) and loss-of-function (33)(34)(35) mutations of other genes have been found to cause epilepsy. We expect that both types of mutations in CPA6 could lead to epilepsy; however, in vitro experiments present difficulties in discriminating between the two, as it is not always clear how these mutations will affect the human brain in vivo (36).…”

Molecular biology and biophysical properties of ion channel gating pores

“…Several mutations in VSDs have been reported to cause hypokalemic periodic paralysis (HypoPP) and normokalemic periodic paralysis (NormoPP), cardiac arrhythmias associated with dilated cardiomyopathy, peripheral nerve hyperexcitability, and epilepsy. Most of the mutations related to these channelopathies are located on the S4 segment of the VSDs of the Na v 1.1, Na v 1.2, Na v 1.4, Na v 1.5, Ca v 1.1, and K v 7.4 channels (Gosselin-Badaroudine et al 2012b, 2013Matthews et al 2009;Misra et al 2008;Volkers et al 2011;Wuttke et al 2007). The biophysical characterization of several of these mutations has revealed the presence of an alternative permeation pathway through VSDs, which are usually non-conductive.…”

Novel SCN1A variants in Dravet syndrome and evaluating a wide approach of patient selection

“…Further studies revealed that loss-of-function variants found in DS patients and studied in mouse models induce hyperexcitability and decrease of Na + current in neocortical (Ogiwara et al 2007) and hippocampal (Yu et al 2006) γ-aminobutyric acid (GABA) ergic inhibitory interneurons. Volker and colleagues, on the other hand, performed and described a biophysical analysis of an identified variant, which surprisingly causes DS by overall gain of function (Volkers et al 2011). It should also be mentioned that in some cases missense variants in SCN1A can also be associated with DS (Ohmori et al 2002;Fukuma et al 2004); however, we did not identify possibly pathogenic Clinical significance was determined according to dbSNP database (Sherry et al 2001); N/A, not available.…”